With the recent rapid improvement in the performance of electric devices, more functional and economical printed wiring boards are needed. For substrate materials, the combination of economical glass fibers having superior electric insulating properties, strength, etc. with an epoxy resin is generally used. However, the use of glass fibers cannot keep up with recent trends, such as the increase in wiring density and the development of chip mounting. That the use of copper-clad laminates (or aramid substrates) in which an aromatic polyamide fiber nonwoven fabric cloth is applied as a substrate has been suggested (see, for instance, U.S. Pat. No. 4,729,921, Kokai (Laid-open) Japanese Patent Application No. Sho 60-52937, No. Sho 61-160500, No. Sho 62-261190, No. Sho 62-273792, No. Sho 62-274688 and No. Sho 62-274689).
Because of characteristics such as low expansion, low dielectric constants and light weight, the application of the aramid substrates to electric devices has been examined for public, industrial and defense purposes.
However, the above-mentioned aramid substrate is not useful because it has a substantial amount of impure ions and a large absorption factor. The previously known aramid substrates have a poor insulating reliability due to the lack of adherence between the nonwoven fabric cloth and impregnated resin. The aramid substrate is also susceptable to mechanical deformation such as by warping and twisting. In order to lessen these problems, improvements in aromatic polyamide fibers, use of a binder for the aromatic polyamide fibers, and the nonwoven fabric cloth surface quality and texture have been tested. Furthermore, the development of impregnated resins has also been tested.